Diatom-based reconstruction of Holocene hydrological changes along the Colorado River floodplain (northern Patagonia, Argentina)

FAYO ROCÍO; ESPINOSA M. A.; VELEZ AGUDELO CAMILO; PAN JERONIMO; ISLA FEDERICO I.

JOURNAL OF PALEOLIMNOLOGY

SPRINGER

Lugar: Berlin; Año: 2018

0921-2728

Diatom assemblages recovered from a Colorado River delta core in northernPatagonia, Argentina were analyzed in order to assess past environmental conditions. A total of 35 samples were selected from the 172-cm core extracted 37 km from the mouth (39º36´19.6 "S; 62º29´26.1" W). 138 taxa were recognized and grouped according to life forms. Two diatom zones were identified by cluster analysis. At the base of the sequence, the Diatom Zone I (DZI; 4132±35 - 2919±27 14C yr BP), presented clays and was characterized by assemblages dominated by aerophilous and benthic taxa and Chrysophyte stomatocysts that allowed to infer a sedimentary environment corresponding to a pond experiencing dry periods. The upper section (DZII) was dominated by fine sands and silts encompassing the last ~150 yr with abundant planktonic and tychoplanktonic diatoms. Benthic diatoms were abruptly replaced by planktonic forms in this zone indicating deeper waters. These results characterize the meandering dynamics of a deltaic system. During the Mid-Holocene,more arid periods would have favored the deactivation of meanders and the formation of riverine and oxbow wetlands. In Late-Holocene and historical times, more humid conditions and the hydrological system across the floodplain reactivated the paleochannel. Today, the dominant diatom species are brackish/freshwater fragilaroids. A non-multidimensional scaling analysis (NMDS) showed a lack of analogy between fossil and modern samples. The change in diatom floras in recent historical times was attributed to anthropic disturbances, a consequence of the regulation of the river flow. This regulation is evidenced by a less discharge, morphological modifications in the floodplain and increased salinity in the last decade.